Novel α-substituted tropolones promote potent and selective caspase-dependent leukemia cell apoptosis

Abstract

Tropolones, such as β-thujaplicin, are small lead-like natural products that possess a variety of biological activities. While the β-substituted natural products and their synthetic analogs are potent inhibitors of human cancer cell growth, less is known about their α-substituted counterparts. Recently, we synthesized a series of α-substituted tropolones including 2-hydroxy-7-(naphthalen-2-yl)cyclohepta-2,4,6-trien-1-one (α-naphthyl tropolone). Here, we evaluate the antiproliferative mechanisms of α-naphthyl tropolone and the related α-benzodioxinyl analog. The α-substituted tropolones inhibit growth of lymphocytic leukemia cells, but not healthy blood cells, with nanomolar potency. Treatment of leukemia cell lines with the tropolone dose-dependently induces apoptosis as judged by staining with annexin V and propidium iodide and Western blot analysis of cleaved caspase 3 and 7. Moreover, pre-treatment of cells with the caspase inhibitor Z-VAD-FMK inhibited the apoptotic effects of the tropolone in two lymphocytic lines. Caspase inhibition also blocked elevated histone acetylation caused by the tropolone, indicating that its effects on histone acetylation are potentiated by caspases. In contrast, α-naphthyl tropolone upregulated p53 expression and phosphorylation of Akt and mTOR in a manner that was not rescued by caspase inhibition. The effects of tropolone were blocked by co-incubation with high levels of free extracellular iron but not by pre-loading with iron. Additionally, dose and time dependent reduction in ex vivo viability of cells from leukemia patients was observed. Taken together, we demonstrate that α-substituted tropolones upregulate DNA damage repair pathways leading to caspase-dependent apoptosis in malignant lymphocytes.

Novel tropolones induce the unfolded protein response pathway and apoptosis in multiple myeloma cells

“…We found that MO-OH-Nap treatment inhibits proliferation and induces caspase cleavage in myeloma cells, consistent with previous studies in leukemia cell lines where α-tropolone derivatives promoted apoptosis through caspase activation in a concentration-dependent manner [ 6 ]. Both the classic intrinsic (caspases 9 and 3) and extrinsic (caspases 8 and 3) apoptotic pathways are involved.…”

“…Currently under development are HDAC-6-specific inhibitors, as synergy has been observed with proteasome inhibitors [ 28 , 29 ]. Selective tropolone analogues have previously been shown to have effects on histone acetylation and to have selective cytotoxic effects for malignant cells compared to healthy human peripheral blood mononuclear cells [ 6 , 13 ]. Here we have demonstrated that MO-OH-Nap, a novel α-substituted tropolone, induces apoptosis and the UPR in myeloma cells.…”

Efficacy and cytotoxicity in cell culture of novel α-hydroxytropolone inhibitors of hepatitis B virus ribonuclease H

“…However, potency dropped about five-fold (EC 50 = 15 µM) when this methyl group was instead a methoxymethylene (#108) (Table 1). A similar decrease in activity from #106 to 108 was also observed against the HIV RNaseH, where enzymatic IC 50 values were 0.23 and 1.0 µM, respectively (Murelli et al, 2016). On the other hand, when R 1 was instead a chloromethylene (#107), a dramatic increase in anti-HBV activity was observed, with an EC 50 value of 0.4 µM, which is six-fold more potent than #106.…”

“…Troponoids have powerful antibacterial and antifungal activities (Baillie et al, 1950; Zhao, 2007). In addition, many other biological effects such as antiviral (Boguszewska-Chachulska et al, 2006; Budihas et al, 2005; Miyamoto et al, 1998b), antitumor and antiproliferative (Li et al, 2016; Miyamoto et al, 1998a; Oblak et al, 2012), and anti-inflammatory (Rekka et al, 2002) activities are associated with troponoids. Thus, troponoids hold promise to be developed into new medicines for a range of diseases.…”

Biosynthesis of Tropolones in Streptomyces spp.: Interweaving Biosynthesis and Degradation of Phenylacetic Acid and Hydroxylations on the Tropone Ring

“…The contiguous array of at least three oxygen atoms (ketone/hydroxyl) and the sevenmembered aromatic ring are both essential for the broad bioactivities of compound 1 (6,7). Interest in the potential bioactivities and unusual structure of compound 1 has generated a number of chemical synthesis studies focused on the derivatization of tropolone, as well as studies supporting tropolone as a very promising therapeutic drug lead (2,(8)(9)(10).…”